1. Field of the Invention
The present invention relates to foam insulated water heaters and methods of making same. More particularly, the present invention relates to a method of insulating a water heater tank with an expandable foam insulation material such as polyurethane foam.
2. Description of the Prior Art
The advantage of using rigid polyurethane foam insulation in water heater construction has been recognized for several years. For example, the heat conductivity of polyurethane foam is lower than that of fiberglass, thereby providing superior insulation properties. Thus, it is possible to obtain the same insulation properties utilizing a substantially reduced insulation wall thickness, as compared to conventional insulation materials, such as fiberglass. This results in water heaters having a reduced size thereby providing lower packaging and shipping costs.
In addition, the rigidity of the foam insulation, when compared with that of fiberglass, provides improved resistance to dents in the exterior jacket of the tank. This factor permits the use of less sophisticated (and therefore less expensive) shipping containers.
Although the superior insulating properties of expandable foam materials such as polyurethane have been well recognized for many years, the use of foam as an insulating material in water heaters has been to date quite limited. This is due at least in part to the production problems encountered using expandable foam materials. One of the major problems associated with water heater manufacturing, and particularly the production of foam insulated water heaters, has been the method by which the foam insulation layer is formed about the tank. Generally, the foam is injected as a liquid which continually expands and eventually sets into a rigid foam layer. Usually the liquid foam is injected into the annular space between the inner tank and the outer jacket. Unfortunately, the liquid foam has a tendency to leak out of any small openings in the seams of the outer jacket. In addition, the sides of the tank generally are provided with openings such as a drain opening or with valves such as a temperature and pressure release valve. Also attached to the sides of the tank are electric controls and other components such as thermostats. In the past, it has been a serious problem in preventing these openings and components from becoming covered with foam, interfering with subsequent servicing, repair, adjustment, etc.
One way of shielding these areas from the expanding foam has been to pack the regions surrounding the nipples and controls with fiberglass insulation material. The fiberglass insulation material then serves as a "foam dam" during the foaming operation.
Other processes utilize a plastic bag filled with the expanding foam material, the bag being positioned around the tank. In U.S. Pat. No. 4,372,028 the liquid foam material is first injected into a bag. The bag is then sealed and positioned circumferentially or longitudinally about the tank. The bag may have welded cut out openings which fit over any components attached to the exterior wall of the tank. Because the foam is wholly contained within an enclosed plastic bag, there is no danger of the foam leaking into unwanted areas. Once the foam within the bag expands sufficiently, it forms an effective dam about the electrical control or other area. Subsequently, the remaining annular space between the tank and the outer jacket may be foamed without danger of the foam leaking into the components.
U.S. Pat. No. 4,447,377 utilizes another type of plastic bag used in foam insulated water heaters. The bag has a shape which may extend substantially around the entire circumference of the tank. The bag is first positioned around the tank and then the outer jacket is positioned around the bag. Cut out weld holes may be provided in the bag and positioned about any components such as electrical controls, thermostats, drain lines, etc. The top pan is then positioned over the jacket and foam is injected (through an opening in the top pan) into the bag. Since the expanding foam is wholly contained within the bag, there is no danger of unwanted foam leakage.
It has now been discovered that remarkable advantages are achieved by letting the expanding foam material flow freely, without containing it within a bag. It has now been discovered that the expanding foam material gains viscosity and volume as it is being formed and that a surprisingly improved product is obtained by controlling the flow only when the foam is in its most liquid state. Thus, it is an object of the present invention to provide a method of injecting foam insulation into the annular space between a hot water tank and the outer jacket wherein the injected foam is controlled only at the very initial stage of this injection, and is later allowed to freely expand within the annular space.